WASHINGTON — A defiant Iraq has loosed millions of barrels of crude oil into Persian Gulf waters off Kuwait over a period of several days in what the Bush Administration on Friday branded "environmental terrorism" of an "immense and shocking magnitude."

U.S. officials said that Iraqi forces have opened the valves on an oil-loading pipeline at the Sea Island terminal of the Al Ahmadi refinery. The terminal is about 10 miles off the Kuwaiti coast, pouring an estimated 100,000 barrels of slippery brown crude into the strategic waterway each day.

The Iraqis have also emptied the crude from five large tankers that have been moored off Kuwait city since last October, U.S. government spokesmen said. Each tanker held several hundred thousand barrels of oil.

The resulting spill has created "an enormous mess in the Persian Gulf of rather frightening consequences," said Pentagon spokesman Pete Williams. Military officials insisted, however, that the oil spill would have no impact on the progress of Operation Desert Storm.

Iraqi President Saddam Hussein has long threatened to set the sea afire to thwart an allied amphibious assault. But U.S. military planners appear to have been taken by surprise by the spill and were scrambling to assess the size and danger of the spreading oil plume and to find ways to combat it.

Government officials estimated that, as of Friday, the spill involved a dozen times more crude than the Exxon Valdez disaster, the largest oil spill in U.S. history. The Valdez drenched the shores of Alaska's Prince William Sound with 258,000 barrels of crude oil in March, 1989.

"Saddam Hussein continues to amaze the world," President Bush said at a White House news conference.

Although the President and Pentagon officials discounted the impact on military operations, the spill clearly has the potential to force allied warships to alter their sailing patterns, impede amphibious landings and foul the Saudi desalination plants that provide drinking water to the more than 700,000 allied troops in the region.

The United States has 20,000 Marines aboard ships in the Persian Gulf. They are awaiting orders to mount an amphibious assault on Kuwait as part of the allied campaign to drive Iraqi troops from the tiny oil-rich emirate.

Environmentalists expressed outrage at the act, which they said threatens Persian Gulf waters, wildlife, marshes, coral reefs and beaches.

U.S. military officials in Saudi Arabia described the slick as eight to 10 miles long and about two miles wide, moving slowly southward.

It was not known precisely when the oil release began. White House Press Secretary Marlin Fitzwater said it appeared that the oil had been pouring out of the pipeline for several days. He said Bush was briefed on the spill Thursday and again Friday by his national security adviser, Brent Scowcroft.

"The President has been monitoring this for a couple of days," Fitzwater said. "This is a growing problem and a very serious one. . . . We don't have any way of knowing how vast it will be."

Representatives of the Energy Department, the Defense Department, the National Oceanic and Atmospheric Administration and others met again Friday afternoon and scheduled another session for today.

Officials said it is likely that a team will be dispatched to the gulf over the weekend to study the environmental disaster firsthand.

Army Lt. Gen. Thomas W. Kelly, director of operations for the Joint Chiefs of Staff, said that military planners had "done our studies" on the possibility of a deliberate oil spill, but he suggested that current plans did not envision a spill of this magnitude.

"No operations plan ever survives contact with the enemy," Kelly said ruefully at a Pentagon briefing.

The general said the spill is not militarily useful to Hussein's forces and insisted that the U.S.-led coalition participating in Operation Desert Storm "will be able to work around it."

"I can find nothing that would make sense militarily of releasing millions of barrels of oil into the Persian Gulf," Kelly said. "It is the act of an international terrorist."

But military analysts said the spill would limit operating room within the narrow gulf, perhaps forcing the United States to move its two aircraft carriers south, taking them farther from their targets in Iraq and Kuwait.

Fitzwater acknowledged that the United States had no specific plan for dealing with the spill "at this point," and said that military activities would be adjusted to cope with it.